1. Field of the Invention
The present invention relates generally to the maintenance, testing and upgrading of large diameter fluid transmission lines, and in particular, to a method and apparatus for isolating a component such as a main line valve without evacuating the fluid product from the transmission line.
2. Description of the Prior Art
Large diameter pipelines have been used for many years to transport crude oil, gasoline, jet fuel, coal slurry, natural gas and other petroleum derivatives and products, many of which are very flamable and dangerous to the ecology. The liquid products are gathered from the field and are transported through the pipelines to refiniers and processing plants. The finished products from the refineries and processing plants are transported through other pipeline systems over long distances to market.
These pipelines must be operated at high pressures in order to economically transmit large volumes of product. The high operating pressures represent a significant percentage of the maximum pressure which the pipe will withstand before rupture or leak occurs, even if the pipe is in perfect condition. Because of the nature of the products typically transported through a large diameter pipeline, rupture of the pipeline can seriously damage the ecology and endanger human life. New pipelines may leak or rupture for a number of reasons, including faulty manufacture of the pipe, faulty weld joints, or damage to the pipe during transportation or installation. Pipelines which have been in use for some period of time are also subject to leak or rupture due to corrosion.
The typical pipeline is several hundred miles long and is usually divided into sections, each section being separated by a main line valve. Many of the transmission lines have gathering lines for collecting additional product along the route of the principal transmission line. Because of the corrosion and damage which occurs as the liquid product is transported, many of the main line valves or other pipeline components become inoperative. Because of the serious consequences and relatively high likelihood of ruptures and leaks, the components must be repaired before other damage occurs. Furthermore, as different products are transported through the pipeline, modification of the pipeline components is usually required to accommodate new operating conditions such as increased operating pressures. Also, pipeline systems must be updated from time to time in order to comply with minimum safety standards as defined by government regulations.
The maintenance and upgrading of existing pipelines has become increasingly more costly. The principal factors contributing to this increasing cost are labor, equipment and loss of revenue to the operating companies during the shutdown time of the pipeline when repair or modifications are being made. Although the contractors who perform the repair and maintenance have increased their efficiency through the use of improved equipment, the methods of testing, repair and modification have remained basically unchanged. The conventional method for performing upgrading repairs or modifications of liquid product transmission lines is to evacuate the product from the pipeline, cut the line, install test manifolds, locate the component to be repaired or modified, and then install a new component which has been repaired or modified. In performing the conventional repair procedure, it is necessary to shut down the pipeline system for long periods which results in substantial costs to the owner companies, and includes not only the costs of repair and modification, but also the loss of revenue for the time the pipeline is out of service. The shutdown of a major product pipeline may cause the shutdown of the processing plant or a refinery with consequential adverse effects upon markets which are unable to operate without a steady supply of the refined product.
Because of the substantial expense involved and serious impact upon markets which are unable to operate when the pipeline must be taken out of service for a substantial period of time, methods for testing, repairing or modifying existing pipelines are continuously being improved. One such improved method in which a slug of water is utilized instead of test manifolds is disclosed and claimed in U.S. Pat. No. 3,827,282 issued to Beryle D. Brister on Aug. 6, 1974. However, testing, repair or modification of a pipeline according to that improved method requires that the pipeline be taken out of service and the product stripped from the pipeline and displaced by a slug of water. A short section of the pipe containing the slug of water is frozen to seal off the pipeline. Only then can the pipeline be safely tested for leaks or cut to make repairs or modifications. The time required to strip the line, fill it with water, and refill the line with the product after testing or repair is significant and represents a substantial cost. Furthermore, an adequate natural supply of surface water is often unavailable in arid regions.
Although the introduction of a slug of water into a transmission line can be used effectively for certain applications, for example in pipelines which transport crude petroleum or coal slurry, there are a number of applications involving the transportation of refined petrochemical products such as zylene and polypropylene which must be maintained in moisture free conditions to prevent contamination of the liquid product. Also, in artic regions it is important to keep moisture out of the pipeline because frozen residual moisture causes damage to many components such as valves. IIn general, any finished product whether it be diesel, gasoline, jet fuel or natural gas must be kept as dry as possible. Special equipment including dewatering pigs and moisture traps are employed to maintain the pipelines moisture free. Because of the general unavailability of water in arid regions, freezing temperatures in artic regions and because of the necessity to avoid the exposure of some products to moisture, alternate methods must be used to seal the transmission lines for these applications.